The present invention relates to a method and an apparatus for activating a semiconductor impurity implanted in silicon carbide (SiC) and the like, for example, such a method and apparatus required in manufacturing semiconductor devices.
In a production of semiconductor devices utilizing silicon (Si), which is the most popular semiconductor material at present, generally, after adding an impurity in Si by an ion implantation and the like method, the Si is heated to 900xc2x0 C. to 1100xc2x0 C. with the use of an electrical furnace, a flash lamp annealer, and the like, to activate the impurity in the Si.
In recent years, a semiconductor device utilizing silicon carbide (SiC) has drawn considerable attention in the industry since such a device is excellent in electric power characteristics (high breakdown voltage and high current-carrying capacity), high-frequency characteristics, and resistance in an environment of use. However, the ion implantation and activation of SiC involve many difficulties in comparison with those of Si. In order to overcome such difficulties, several techniques in the impurity activation have been suggested. An example of such techniques is that an impurity is added when forming an SiC film, an ion implantation is carried out under a high temperature of about 500xc2x0 C. to 1000xc2x0 C., and thereafter, as disclosed in T. Kimoto, et al., Journal of Electronic Materials, Vol. 25, No. 5, (1996) pp. 879-884 etc., an impurity is activated by a heat treatment at a high temperature of 1400xc2x0 C. to 1600xc2x0 C.
However, such methods of impurity activation by a heat treatment requires a step of heating Si and the like semiconductor material with the use of electrical furnace and the like. Consequently, a relatively long time is necessary for the activation, and therefore it is rendered difficult to increase the productivity. Such drawbacks become more conspicuous in the case of using SiC since a further higher temperature is required in the heat treatment. Moreover, in the case of SiC, regarding a p-type dopant, it is difficult to form a semiconductor layer in which the p-type dopant element is activated to a high degree.
In view of such drawbacks, for example, Japanese Unexamined Patent Publication No. 7-022311 discloses such a method of an impurity activation as described in the following. According to this, a laser annealing is conducted by irradiating with a laser light an amorphous Si film in which concentrations of carbon, nitrogen, and oxygen are made to be lower than certain values, in order to form a mixed region in which an amorphous region and a solid-phase ordered region are present together without fusing the amorphous Si film. Then, impurity ions are implanted into the amorphous Si film, and thereafter laser annealing is carried out by irradiating the Si film with a laser light having a wavelength of 248 nm to make an impurity region to be a semi-amorphous state. However, although it is disclosed in the Publication No. 7-022311 that a carrier mobility can be improved by the method when compared with an amorphous Si, a laser annealing for the semiconductors other than the amorphous Si is not mentioned.
A laser light conventionally used for a laser annealing for such a crystallization (activation) of a semiconductor has been a laser light having a wavelength shorter than a wavelength causing a band edge absorption, such as an excimer laser, as described specifically in Y. Morita, et al., Jpn. J. Appl. Phys., Vol. 2, No. 2, (1989) pp. L309-L311. In the case of using a laser light having such a wavelength, electrons in the atoms constituting a semiconductor are excited and ionized by the energy of the laser light, and part of the energy of the electrons is converted into a lattice vibration of the atoms, transiently heating the semiconductor to a high temperature and thus promoting the crystallization (activation) of the semiconductor.
However, in such a prior art impurity activation by a laser annealing as described above, a laser apparatus with a relatively large output power is required since efficiency in energy utilization is low, and therefore the manufacturing cost tends to be increased. Furthermore, according to such a method, it is not easy to carry out the activation of impurity with high reliability and to produce semiconductor devices with desirable characteristics. In particular, the production of semiconductor devices with desirable characteristics is difficult in the activation of p-type impurities in the case of SiC.
In view of the foregoing drawbacks in prior art, it is an object of the present invention to provide a method and apparatus for activating a semiconductor impurity in which the activation of the impurity can be carried out with high efficiency and reliability even when a laser apparatus with a relatively small output power is used.
This and other objects are accomplished in accordance with the present invention by providing a method for activating a semiconductor impurity in a semiconductor comprising a major semiconductor element and an impurity element by irradiating the semiconductor with a light, the light having a longer wavelength than a wavelength causing a band edge absorption of the semiconductor. The light may be a light having such a wavelength that a resonance absorption is caused by a characteristic vibration in a bond of the major semiconductor element and the impurity element.
In the cases of prior art activation methods utilizing a light having a wavelength shorter than a wavelength causing a band edge absorption of a semiconductor, electrons in the atoms constituting the semiconductor are excited and ionized by the energy of the light, and part of the energy of the electrons is converted into the energy for a lattice vibration of the atoms. The semiconductor is thereby heated transiently to a high temperature, and thus the impurity is activated. On the other hand, the present inventors have found that, by irradiating a semiconductor with a light having a longer wavelength than a wavelength causing a band edge absorption of the semiconductor, a lattice vibration between the impurity element and the semiconductor element can be directly caused, and thereby the impurity can be activated. Therefore, according to the present invention, such advantageous effects are achieved that the efficiency in the activation is made to be excellent, that a laser apparatus with a small output power can be employed, and that a desirable impurity activation can be readily carried out.
More specifically, for example, in the cases where the major semiconductor element is silicon carbide and the impurity element is one of aluminum, boron, and gallium, a light having a wavelength of 9 xcexcm to 11 xcexcm, which is longer than a wavelength causing the band edge absorption (in the case of 6Hxe2x80x94SiC, approximately 3 eV: up to 0.41 xcexcm), may be employed, in order to readily produce a p-type silicon carbide semiconductor with desirable characteristics. In particular, in the case of aluminum, it is more preferable to employ a wavelength of 9.5 xcexcm to 10 xcexcm.
According to another aspect of the invention, in irradiating a semiconductor with a laser light having such a wavelength as described above, the laser light may be focused on a focal point adjacent to a surface of the semiconductor, and the focal point of the laser light may be made to be a point between a light source of the laser light and the surface of the semiconductor having a predetermined distance from the surface of the semiconductor. More specifically, in irradiating a semiconductor with a laser light having such a wavelength as described above, the laser irradiation may be carried out by detecting a plume caused in the case where the focal point of the laser light is brought to a position adjacent to the surface of the semiconductor from a direction of the light source of the laser light, and controlling the focal point of the laser light to be such a position that the plume starts to be detected.
By setting and controlling the focal point as described above, the degree of the activation is further improved easily.